In this type of internal gear pump, a ring-shaped internally toothed gear provided with a plurality of internal teeth is rotatably accommodated in a housing hole of a pump housing. An externally toothed gear provided with a plurality of external teeth which internally mesh with the plurality of internal teeth of the internally toothed gear is eccentrically accommodated in the internally toothed gear with respect to the internally toothed gear. The internally toothed gear is rotated by a rotational drive of the externally toothed gear, whereby a liquid is sucked from a suction port into a space defined by the plurality of external teeth and the plurality of internal teeth. The liquid is discharged from a discharge port through the space.
The shape of individual external teeth of the externally toothed gear is designed using a base circle and a rolling circle rolling without slipping around the base circle. Specifically, a fixed point is provided at a position spaced from a center of the rolling circle by an eccentricity between a center of the externally toothed gear and a center of the internally toothed gear. A trajectory (curve) drawn by the fixed point when the rolling circle rolls without slipping around the base circle is a trochoid curve. Then, a circle having a predetermined radius and having its center on the trochoid curve is drawn. An envelope of the circle forms the shape of individual teeth of the externally toothed gear.
The internal gear pump as such is described in, for example, Patent Document 1 and so on.